A flow battery is a form of rechargeable battery in which electrolyte containing one or more dissolved electroactive species flows through an electrochemical cell that converts chemical energy directly to electricity. Additional electrolyte is typically stored externally, generally in tanks, and is usually pumped through the cell (or cells) of the reactor by a pump. Flow batteries may be rapidly recharged by replacing the electrolyte liquid (in a similar way to refilling fuel tanks for internal combustion engines). The spent electrolyte materials may be recovered for re-use.
The use of external storage allows independent scale up of power and energy density specifications. External storage of reactants also avoids self-discharge that is observed in other primary and secondary battery systems.